Ultimate equation is pie in the sky, says Hawking

For many scientists, it was the ultimate quest: the hunt for a mathematical equation that described, in principle at least, everything in the universe.

But it seems a "theory of everything" may forever be beyond our grasp. Stephen Hawking, the Cambridge University physicist who suggested in A Brief History of Time that scientists were on the verge of discovering such a theory, has changed his mind.

We may have to make do with never understanding the universe completely, he says.

A theory of everything would be the ultimate in scientific determinism. If we knew the position and velocity of every single particle in the universe, and understood the laws of physics that governed them, we could - given enough computing power - work out the state of the universe and everything in it, at any time we chose. Or so the argument goes.

So powerful would the equation be, that to know it would be to know the mind of God, Professor Hawking famously remarked.

His decision to abandon the hunt for a theory of everything is outlined in a paper posted online.

In Gödel and the End of Physics, Prof Hawking describes how ideas put forward by the late mathematician Kurt Gödel all but rule out a theory of everything.

Gödel developed a series of mathematical paradoxes that could not be proved. If there were such mathematical conundrums, then there must also be physical problems, such as understanding the universe, that will also be beyond us, Prof Hawking argues.

He admits his change of heart will disappoint many.

"Most people have implicitly assumed that there is an ultimate theory that we will eventually discover. Indeed, I myself have suggested we might find it quite soon," he writes.

But he remains upbeat. Knowing some things will forever remain unknown will keep scientists motivated. "I'm now glad that our search for understanding will never come to an end, and that we will always have the challenge of new discovery. Without it, we would stagnate."

Sir Martin Rees, the astronomer royal, said a "theory of everything" had long been considered unlikely. "In some sense, there must be such a theory, but its nature and what it will imply is very controversial," he added.